Windows or glass areas are a significant weakness in heat insulation schemes for buildings in hot or cold climates. A basic insulating window that is well-known is constructed from two panes of glass within a rigid frame. The air space between the panes provides heat insulation. It is also known to evacuate the air space or to fill the air space with a gas of lower thermal conductivity than air such as argon. One further method of enhancing the insulating value of such a window is to increase the air space and provide transparent partitions between the outer glass panes to reduce convective heat transfer within the unit.
Other technologies include providing selectively reflecting or low-emissivity coatings to reduce radiant heat transfer through the window. As well, there have been significant improvements in the window frame, both in the union of the glass panes and the design and material of the frame. The layers of glazing in an insulating unit must be held apart at the appropriate distance by spacers. Because of its excellent structural properties, window manufacturers have used aluminum spacers. Unfortunately, aluminum is an excellent conductor of heat and the aluminum spacer used in most standard edge systems represented a significant thermal “short circuit” at the edge of the insulating glass unit, which reduces the benefits of improved glazings. In addition to the increased heat loss, the colder edge is more prone to condensation.
A window comprising a compound glass element with an edge binding member is described in U.S. Pat. No. 5,260,112 (corresponding to Canadian Patent No. 2,029,148). The edge binding member holds two glass planes apart in a parallel condition and seals the space between the glass panes from the exterior. A suitable edge binding member must provide a stable mechanical bond between the glass panes to ensure the physical integrity of the window unit. The edge binding member must also be vapour tight to prevent the penetration of vapour between the glass panes and consequent condensation that will occur within the window unit. Lastly, the edge binding member must not provide a thermal bridge between the glass panes, or should at least minimize heat transfer from one pane to the other. With the edge binding member of this patent, a metal foil band wraps around a spacer and substantially bridges the glass panes. The spacer is glued to the glass panes with an acrylate adhesive. At the outer edges of the metal band, close to the glass panes, the spacer is bevelled, creating a triangular void space. The void space is filled with a highly vapour resistant hot-melt butyl adhesive.
While this system does provide a solution to the vapour block problem, a significant additional problem arises. The spacer is typically made from a thermoplastic material with a significantly higher thermal coefficient of expansion that the steel band. The butyl adhesive tends to creep into the space between the plastic spacer and the metal band, thereby adhering the two together. When subject to thermal expansion or contraction, the different rates of expansion between the plastic and the metal may cause the spacer to fail, or it may disrupt the integrity of the butyl seal.
Therefore, there is a need in the art for an energy efficient window unit which includes a spacer and seal system which mitigates the difficulties posed by the prior art.